The present invention relates to calibration of analog and mixed-signal integrated circuits (ICs) and particularly to that of various classes of amplifiers.
Amplifiers have formed an important class of analog electronics circuits since the technology was first practiced. They continue to be so today, even in highly integrated circuits. A signal to be amplified is presented at an amplifier's input terminal(s), and an amplified version of that signal is available from the amplifier's output terminal(s). “Amplified” in this context means “with higher power”, where the input and output signals do not necessarily need to be carried by the same physical quantity, such as voltage or current. Therefore, an amplifier may make a conversion from one physical quantity to another. On the other hand, a passive device that merely scales an input signal without amplifying its power, such as a transformer, is not considered an amplifier. Amplifiers often employ a form of feedback of the amplified signal at or near the output terminals to the input terminals. This feedback can have a negative or positive polarity, and is therefore referred to as negative or positive feedback. Negative feedback is often used when an amplifier needs to have a high precision, as negative feedback reduces errors. Positive feedback may be used when an amplifier needs to have a high gain, as positive feedback reinforces the input signal. Occasionally, amplifiers employ feed-forward instead of feedback. This technique is used as an alternative to negative feedback for improving linearity. It provides for unconditional stability. However, whichever technique is used, amplifiers can suffer degradations when implemented in the smaller modern IC fabrication technologies.
FIGS. 1A-C illustrate problems that may cause performance loss in amplifiers of all types. FIG. 1A shows case 100A, where an increase of a differential input voltage ΔVin results in an increase of output signal (in this case a current) Io2 with a concurrent decrease of output signal Io1. At zero differential input voltage ΔVin, the output signals Io1 and Io2 are equal. The level ICM at which Io1 and Io2 are equal is usually called the common-mode current. Although the curve of Io2 as a function of ΔVin is approximately straight near ICM, it shows non-linearity for larger values of ΔVin. To prevent distortion, signals need to be kept small. This may be a problem when high efficiency is needed.
FIG. 1B shows a case 100B in which Io1 and Io2 equal ICM at a non-zero differential input voltage ΔVin=Voffset. This can for instance be the case when transistors in input circuits are of unequal size or drive strength. A lower offset voltage Voffset is generally a measure for a better balanced amplifier. It generally also results in better accuracy.
FIG. 1C illustrates a case 100C in which hysteresis occurs. Output signals Io1 and Io2 follow a different curve for a decreasing differential input voltage ΔVin than they follow for an increasing differential input voltage ΔVin. As a result, the common mode current ICM is achieved at differential input voltages ΔVin that depend on the signal history. The widest difference is called the hysteresis width. Hysteresis is caused by memory effects, such as may occur in a poorly parameterized amplifier with positive feedback, and it may result in inaccuracy as well as loss of speed.
The above problems were illustrated based on the example of a transconductance amplifier, which has an input voltage signal and an output current signal. However, they are equally valid for other types of amplifiers, including but not limited to voltage amplifiers, current amplifiers, and transimpedance amplifiers.
FIG. 2 illustrates an example of an amplifier 200 with positive feedback. It is commonly nicknamed the Nauta amplifier after inventor Bram Nauta, who first published about It in 1989 (Bram Nauta, Evert Seevinck, “Linear CMOS Transconductance Element for VHF Filters”, Electronics Letters vol. 25 no. 7, March 1989). Nauta and Seevinck, incidentally, referred to an independent earlier publication of the circuit in U.S. Pat. No. 3,991,380 by Richard Lee Pryor, in 1976. Amplifier 200 is a transconductance amplifier, converting a differential input voltage between its input terminals 202 and 204 to a differential output current between its output terminals 206 and 208. Its gain, or rather transconductance, is defined as the differential output current divided by the differential input voltage. Ideally, it cancels any effects of a common mode voltage at the input.
This amplifier is useful in, for instance, complementary metal oxide semiconductor (CMOS) IC designs because it can combine a high gain with a broad bandwidth and large relative input and output signal ranges.
Amplifier 200 includes input buffer inverters 211 and 212, cross-coupled inverters 213 and 216, and self-coupled inverters 215 and 214. Cross-coupled inverters 213 and 216 provide amplification and positive feedback. Dependent on the balance of relative drive strengths of input buffer inverters 211 and 212 with cross-coupled inverters 213 and 216 and self-coupled inverters 215 and 214, the circuit may show a fixed relationship between input and output signals, or it may show a hysteresis. The maximum gain is also strongly dependent on the relative strengths of these buffers. To achieve optimum performance, the amplifier is designed and operated at a point close to where hysteresis would start occurring.
The relative drive strengths of the inverters 211-213 and 216 in CMOS technology is, among other factors, determined by the relative sizes of transistors in those inverters. If the relative drive strengths deviate from the ideal case, an amplifier may show inferior performance or undesired non-linearity due to the hysteresis. Mismatch of the inverters will reduce the maximum achievable gain as well as the rejection of common mode signal components. In older IC technologies it was relatively easy to obtain accurately matching transistors. In today's circuits where transistors with features much smaller than the wavelength of visible light are etched based on exposure with near-visible light, accurate size and matching are exceedingly difficult to achieve. Therefore, calibration is required to operate an amplifier in or near its optimum point.
Prior calibration circuits, such as proposed by Nauta himself, relied on matching to a replica, using duplicate structures. This type of calibration or tuning cannot operate in today's advanced technologies, because matching is no longer accurate.
A transconductance amplifier that can be calibrated has been proposed in U.S. Pat. No. 8,988,148 by Julian Jenkins, Torsten Lehmann, and James Koeppe. Among other innovations, their invention taught adding a control port to at least one of the inverters commonly seen in integrated amplifiers. However, the document did not specify a calibration method.
The present invention fills this void and teaches new calibration methods and circuits that will enable optimization of all types of amplifiers, including those exhibiting hysteresis, even in advanced IC production technologies.